Hydroponics was the growing of plants in a medium other than soil, such as a nutrient-rich liquid.
Enterprise NX-01 maintained a hydroponic greenhouse to provide fresh fruits and vegetables for the crew's mess hall. (ENT: "Breaking the Ice") The Y-class cargo freighter ECS Fortunate also maintained a hydroponics section. (ENT: "Fortunate Son")
Some Federation starships had laboratories devoted to hydroponics. The USS Enterprise-D had a hydroponics laboratory, which was to be visited by the winners of the primary school science fair in 2368. (TNG: "Disaster") The USS Enterprise-E had a hydroponics laboratory on Deck 11. (Star Trek: First Contact) Deep Space 9 also had a hydroponic garden. (DS9: "Blaze of Glory")
The USS Voyager did not have a hydroponics lab, but the variable environmental controls in cargo bay two could support hydroponics equipment. Shortly after Kes joined the crew in 2371, she volunteered to establish a hydroponics bay in cargo bay two in order to provide fresh fruits and vegetables. She used nitrogenated soil instead of an alternative for Voyager's hydroponics lab. (VOY: "Parallax")